Extra- and intracellular signaling pathways regulating nodule formation by hemocytes of the silkworm, Bombyx mori(Lepidoptera: Bombicidae)

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It has been suggested that early defense mechanisms of the silkworm Bombyx mori, which can clear 106 bacterial cells from the hemolymph within 30minutes, largely depend on pathogen recognition mediated by dual-carbohydrate recognition domain (CRD) C-type lectins and nodule formation by hemocytes. In order to understand this phenomenon, the present study explores both inter- and intracellular signal transduction pathways in association with nodule formation by hemocytes of B. mori. The C-type lectin inhibitors dithiothreitol and ethylenediaminetetraacetic acid inhibited in vivo nodule formation, and a dual-CRD C-type lectin was required for in vitro nodule-like aggregation. These results suggested that dual-CRD C-type lectins were required as pathogen-recognition receptors for nodule formation. Serine protease inhibitors such as benzamidine hydrochloride, aprotinin, and leupeptin inhibited nodule formation, and thrombin, a serine protease, induced in vivo nodule-like aggregation. This indicates that serine proteases behave as mediators in the extracellular signal cascade leading to nodule formation. An inhibitor of phospholipase C, U-73122, inhibited both in vivo nodule formation and in vitro nodule-like aggregation. EDTA and pertussis toxin inhibited in vivo nodule formation. These results suggest that the cell reaction triggered by the two G protein-mediated signal transduction pathways plays a significant role in nodule formation.